A compressor of a gas turbine engine typically has a row of inlet guide vanes and plural compressor stages, each stage comprising a set of stator vanes which receive and redirect the working fluid issuing from the rotating blades of the preceding stage. As aero engines have to operate at varying speeds and inlet conditions, it can be advantageous to be able to vary the geometry of the compressor e.g. to alter the aerodynamic flow angle of individual inlet guide vanes and stator vanes within the gas turbine annulus and/or to adjust compressor bleed valve settings, depending on the engine operating speed and conditions. Vanes whose flow angles are alterable in this way are known as variable vanes.
A large variety of systems are conventionally used for varying compressor geometry. For example, unison rings can be used to rotate variable vanes about their radial axes and thereby change the aerodynamic flow angle. Each unison ring encircles the engine and is rotated by one or more actuators to produce movement in the circumferential direction. This movement can be converted by an arrangement of levers and spindles into the rotation of the variable vanes.
It is known to use hydraulic actuators which use fuel as their hydraulic fluid (i.e. “fueldraulic” actuators) to control the compressor variable geometry. However in recent years there has been an increase in the level of actuation required in gas turbine engines, i.e. increases in both the number of actuators and the loads they have to produce.
One option for adapting fuel systems to meet future higher load requirements is to use larger fueldraulic actuators and larger servo-valves in conjunction with a higher minimum fuel pump pressure rise. However, raising the minimum pump pressure rise increases heat input to the fuel and increases the risk of damage to the pump bearings, which have to run on thinner bearing film thicknesses at high pressure, low speed conditions.
Another proposal, therefore, is to provide a separate fuel pump, such as an axial piston pump, to provide pressurised fuel to actuate auxiliary engine devices such as variable vanes and bleed valves. An example of an axial piston pump in such a fuel system is described in US 2015/0192075. However, this proposal significantly increases the complexity of the fuel system.